Azeotropic distillation of five carbon diolefins in the presence of methyl formate



Patented Sept... 2, 1947 UNlTE AZEOTROPIC DISTILLATION OFFIVE CAB- BON DIOLEFINS IN THE PRESENCE OF METHYL FORMATE Theodore W. Evans, Oakland, Rupert 0. Morris,- Berkeley, and Edward C. Shokal, Oakland,

CaliL, assignors to Shell Development Sompany, San Francisco, Calif., a corporation of Delaware No Drawing. Application September 2,1941,

, Serial No. 409,292

9 Claims. (Cl. ace-42) This invention relates to increasing the concentration of diolefins in fluid mixtures containing diolefins and mono-olefins, and particularly to the concentration of isoprene and/or piperylone in fluid mixtures containing these acyclic dioleflns and mono-olefins of such closely adjacent boiling points that practical concentration by ordinary fractional distillation of the mixtures is very difilcult, if not impossible.

When petroleum products are subjected to cracking or pyrolysis, the resulting products, besides the relatively high'boiling hydrocarbons, also contain relatively large quantities of the relatively low boiling unsaturated hydrocarbons which consist of or contain various mono-olefins and diolefins. For example, when straight-run gasoline fractions, e. g. a fraction having a specific gravity 0.8044 an initial boilin tempera-' ture of about 162 C., a maximum boiling temperature of 220 C., and an octane number of 40.5 (as determined by the C. F. R.- motor method), are subjected to pyrolysis at temperatures of between about 700 C. and about 800 0., inthe pres-- ence or absence of reaction-promoting catalysts,

the effluent fractions frequently contain as much as 50% or more of hydrocarbons boiling below about 50 C. Alarge percentage of this relatively low boiling fraction consists of mono-olefins and diolefins containing from two to five carbon atoms per molecule. Such hydrocarbon mixtures may be subjected to fractional distillation to obtain frac. tions each of which consists of, or at least predominates in, hydrocarbons having the same number of carbon atoms per molecule. For instance, by means of an ordinary fractional distillation, it is possible to recover separately at fraction which consists of, or predominates in, hydrocarbons having five carbon atoms per molecule, this fraction containing the various five-carbon atom mono-olefins and diolefins, such as pentenel, pentene-Z, Z-methyl-butene-l, 2-methyl-butene-2z, 3-methyl-butene-l, isoprene, piperylene, cyclopentadiene, etc. It is known that members of each group of hydrocarbons (i. e., those having the same number of carbon atoms per molecule) have boiling temperatures within such a narrow temperature range that practical separation or concentration of any one or more of them by simple fractional distillation is very diiiicult or even impossible. For instance, the following table parts, one of which consists of, or greatly pre-' dominates in the diolefins, while the other pro-- dominates in the mono-olefins.

. 2 gives the boiling temperatures of most of the olefins (i. e., monoand dioleflns) having five carbon atoms per molecule:

Degrees C. 3-methyl-butene-1 20.1 Pentene-l 30.1 Z-methyl-butene-l 31.05 2-methyl-butadiene-l,3 (isoprene) 34.07 Pentene-2 36.4 2-methyl-butene-2 38.4- Cyclopentadiene 41.0 Pentadiene-L3 (piperyleneL; 42.5

This proximity of the boiling temperature of the individual five-carbon monoand diolefins rendersit practically impossible to separateor even concentrate any one or more of them in a hydrocarbon fraction consisting of several or all of these and other five-carbon atom olefins. For instance, as will be seen from an example described hereinbelow', when an olefinic hydrocarbon fraction boiling between 28 C. and 35 C., and containing approximately 34.8% of dioleflns, is sub- .iected even to a careful fractional distillation in the absence of any auxiliary agent, the maximum concentration of the diolefins in any one cut is about 55%-56%, this fraction constituting onLY approximately 7% of the charge subjected to distillation.

It is, however, frequently desirable to effect a more or less complete separation of the hydrocarbons in such oleflnic fractions into at least two For instance, both isoprene and piperylene (whether individually or in combination with each other) are suitable as substitutes for butadiene in various polymerization and co-polymerization processes in which the presence of the five-carbon atom mono-olefins, at least in relativel large percentages, is undesirable. Other instances ofthe desirability or necessity of separating the mono ing ranges have been attempted, none of the containing either or bothof these acyclic diole'flns and mono-olefins havin boiling temperatures which are close to those of the mentioned diolefins by azeotropic distillation with methyl for mate. Another object is to provide n econ omical process whereby hydrocarbon-fractions consisting of substantial quantities of mono-olefins and of diolefins having five carbon atoms per molecule may be subjected to azeotropic distillation with methyl formate to obtain one or more fractions in which the diolefin or diolefins are present in relatively greater concentrations,

According to the present invention, the above and other objects are attained'by subjecting the mixtures containing isoprene and/or pipe'ryle'n'e and one or more of the five-carbon atom monoolefins, to fractional distillation in the presence of a compound which-forms minimum-boiling azeotropic mixtures with the hydrocarbons subjected to treatment, these azeotropic mixtures containing not less than mol percent nor more than 90 mol percent of said azeotropeforming compound. More specifically stated. the present invention comprises a process in which a, relatively narrow boiling hydrocarbon fraction consisting of, or predominating in monoand dioleflns, the subjected to fractional distillation in the presence of methyl formatc;

The quantity of methyl formate to be added to the hydrocarbon fraction to be distilled may vary within relatively wide limits, depending, in part, on the composition of the hydrocarbon mixture to be fractionated andthe distillation equipment employed. Methyl formate, when added to five-carbon atom olefinic fractions, forms minimum-boiling mixtures with each of the monoand diolefins constituting the mixture. In other words, methyl formate causes a lowering of the eilective distillation temperatures ofthe various 'constituents of such mixture. The drop in the distillation temperature, however, is'not uniform for' the different olefins, the methyl formate lowering the distillation temperatures of the five-carbon atom mono-olefins to a greater extent than the distillation temperatures or the diolefins, e. g. isoprene and piperylene. Therefore, the azeotropes of the mono-olefins boil before the azeotropes of the diolefins, thus permit-' ting an economic. efficient and ready method for piperylene present in olefinic hydrocarbon fractions, and particularly those boiling within such a narrow boiling range as to render practical concentration by ordinary or simple fractional distillation very diflicult and uneconomical, if not substantially impossible.

In general, it is preferable to employ the methyl formate in a quantity substantially suflicient to form azeotropes with each and every olefin present in the mixture treated. However, greater or lesser amounts may also be used. If the polar compound is added to a mixture of five-carbon hydrocarbons in an amount only suflicient to form the azeotropesof the mono-olefins present, the fractional distillation of this mixture will permit the removal of the azeotropes containing the mono-olefins, while the diolefins may then be left behind as a residue in the desired concentrated condition. However, because of the .fact that the selective or preferential solvent action of the methyl formate for the dioleflns is exerted more strongly the greater the concentration of the methyl formate, it is usually preferable to'use methyl formate in quantities more than enough to form the azeotropes with the mono-olefins. When so hsed, the distillation of the mixtures effects the concentration of the dioleflns by first producing overhead fractions containing predominantly the mono-oleflns, while theazeotropes of the diolefins with the methyl formatebegin to distill overhead only after substantially all of the mono-olefins have thus been removed in the overhead fraction or fractions. Also, the use of excess amounts of the azeotropeforming methyl formate, besides having no deleterious effect upon the separation of the specified mono-olefins from the dioleflns, is advantageous in that it permits the utilization of'smaller fraction towers, i. e. towers having comparatively .fewer plates.

- The distillation according to the present invention may be effected at atmospheric pressure, although higher or lower pressures may also be used. It must be noted, however, that the use of pressures other than those substantially in the neighborhood of atmospheric may change the ratio of the compounds in the azeotropic mixture. Also, the distillation temperatures forthe azeotropic mixtures containing the different olefins will-change depending on the pressure maintained in the distillation unit.

The present process of concentration of the acyclic diolefins, i. e. isoprene and iperylene, may be applied to any olefinic hydrocarbon mixture containing either or both of these diolefins. However, it is generally preferable to fractionate preliminarily the hydrocarbon mixture, and then to apply the present rocess of azeotropic distillation to the relatively narrow boiling oleflnic fraction or fractions thus produced. For infins, this fraction boiling between about 10 C.

and about 50 C. to 55 C.. However, when such a fraction is distilled in the presence of methyl formats, the fraction predominating in the diolefins will contain both isopreneand piperylene. Also,.probably because of the difierence in the boiling temperatures of the azeotropes containing the methyl formats-and the isoprene or piperylene, the concentration or separation of mixtures thereof from olefinic fractions ontaining both is not attained as readily as when the fractions contain only one of these acyclic diolefins. Therefore, for the purpose of effecting the concentration of these diolefins in the most economic and practical manner, it is generally preferable first to subject the five-carbon atom olefinic fraction to an ordinary fractionation to obtain two relatively narrow cuts, one of which contains the isoprene and mono-olefins having boiling temperatures in the close vicinity of the boiling temperatures in this diolefln, and the other containing piperylene and mono-olefins boiling close thereto. These narrow boiling fractions may then be separately treated according to the present process to obtain cuts consisting of r reat y predominating i the respective diolefins, the degree of concentration of these acyclic diolefins depending, in part, on the efiiciency of the fractionating tower employed. The presence of cyclopentadiene in the fraction subjected to azeotropic distillation according to the present process, is not detrimental to the efllcient concentration of the acyclic diolefin or diolefins present therein. However, since this cyclic diolefin is readily polymerizable even at the distill'ation temperatures employed, it is generally preferable to pretreat the fractions to separate the cyclopentadiene present therein. Such separation may, for example, be effected by heating the fraction to selectively polymerize this cyclic diolefin, separating the polymer thus formed, and, if desired, depolymerizing the polymer to recover the monomeric cyclopentadiene.

The following detailed examples are given for the purpose of illustrating the present process and the modes of executing the invention. These examples are indicative of the advantages derived from efiecting the separation or concentration of the five-carbon atom acyclicv-diolefins according to the present process. It is to be 1111- derstood, however, that the invention is not to be considered as limited to the specific embodiments, modes or onditions of operation disclosed.

Example I The hydrocarbon fraction subjected to treatment consisted of an olefinic fraction boiling between about 28 C. and 35 C., and containing approximately 34.8% of diolefins (mainly isoprene). Approximately 1'75 grams of the fraction were mixed with methyl formate, the mixture containing about 34.6 percent by weight of the hydrocarbons. Approximately 505 grams of this mixture were then introduced into a distillation unit provided with a column containing 15 to 18 theoretical plates. The upper end of the column was in communication with a condenser. The mixture was then subjected to a distillation at atmospheric pressure, the overhead condensate being' collected in a plurality of containers to determine the diolefin content in each of the hydrocarbon fractions or cuts thus obtained. The results of this distillation are presented in the following table:

Hydrocar- Moi per cent out Weight Per Boiling bonsin ofdiolefins Number of charge, cent of range, azeotrope, in solventin grams charge weight free overhead per cent fraction 85 15.6 48 9.5 78 14.1 91 18.0 5 104 20.6 Bottoms 63 12.5 Trap 14 2.6

Example 11 For purposes of comparison, approximately 210 grams of the above-described olefin fraction boilingbetween 28 C. and C. and containing ap-- proximately 34.8% diolefins, were distilled in the same apparatus as that employed in the preceding examples; In this case, however, nopolar 6 compound was added. The results of this distillotion are as follows:

A comparison of the above data shows the superiority of efiecting the concentration of C5 diolefins by subjecting them to distillation in the' presence of methyl formate.

The separation of the methyl formate from the various fractions obtained may be effected, for example, by means of cooling, addition of water, or by a treatment of the azeotropic mixtures in any other convenient manner known to those skilled in the art.

We claim as our invention:

1. A process for separating a mixture of isoprene and mono-olefins having boiling temperatures close tothat of isoprene which comprises adding methyl formate to said mixture in a quantity at least suificient to form minimum-boiling azeotropic mixtures with all of thehydrocarbons, subjecting the mixture thus-formed to fractional distillation to distill all an azeotropic fraction the hydrocarbon content of which predominates in the mono-olefins, continuing the fractional distillation to distill off a fraction comprising an azeotropic mixture the hydrocarbon content of which predominates in isoprene, and separating the isoprene from said last-mentioned fraction.

2. A process of concentrating piperylene in a hydrocarbon mixture predominating in five-car" bon atom diolefins and mono-olefins having boiling temperatures close to that of piperylene which comprises adding methyl formate and subjecting the mixture thus formed to fractional distillation to obtain two overhead azeotropic fractions, the first of which predominates in a minimum-boiling azeotropic mixture of the added methyl formate and the mono-olefins, while the second of said fractions comprises an azeotropic mixture the hydrocarbon content of which predominates in piperylene.

3. A process of concentrating isoprene in a hydrocarbon mixture predominating in five-carbon atom diolefins and mono-olefins having boiling temperatures close to that of isoprene which comprises adding methyl formate and subjecting the mixture thus formed to fractional distillation to obtain at least two overhead azeotropic fractions,

the first of which predominates in a minimumboiling azeotropic mixture of the added methyl formate and the mono-olefins, while the second of said fractions comprises an azeotropic mixture the hydrocarbon content of which predominates in isoprene.

4. A process for separating a mixture of acyclic diolefins and mono-olefins having five carbon atoms per melocule into a fraction predominating in the mono-olefins and a'fraction predominating in the 'diolefins, which comprises adding methyl formate to said mixture in a quantity at least sufficient to form minimum-boiling azeotropic mixtures with all of the hydrocarbons, subjecting the mixture thus formed to fractional distillation to distill off an initial fraction comprising an azeotropic mixture the hydrocarbon content of which predominates in the mono-olefins, continuing the fractional distillation to distill of! a fraction comprising an azeotropic mixture the hydrocarbon content of which predominates in the dloleflns, and separating the hydrocarbons from each 01 said azeotropic mixtures.

5. A process for separating a mixture of acyclic ficient to form minimum-boiling azeotropic mixtures with all or the mono-oleflns in the mixture, and subjecting the mixture thus formed to fractional distillation to distill of! an overhead fraction comprising an azeotropic mixture predominating in the added methyl formate and the mono-olefins.

6. A process of concentrating acyclic dioleflns in a mixture consisting of five-carbon atomhydrocarbons containing dioleflns and mono-olefins which comprises separating cyclic dioleflns from said mixture, adding methyl formate to the remaining mixture in a quantity at least sufficient to form minimum-boiling azeotropic mixtures with the mono-oleflns and thedioleflns in said mixture, and subjecting the-mixture thus formed to fractionat distillation to obtain at least two overhead azeotropic fractions, the first of which contains most of the mono-oleflns present in the hydrocarbons treated, while the second of said fractions comprises a minimum-boiling azeotropic mixture the hydrocarbon content of which predominates in acyclic dioleflns.

7. A process of concentrating diolefins in a mixture consisting of five-carbon atom hydrocarbons and containing dioleflns and mono-oleflns which comprises adding methyl formate to said mixture, said compound being added in a quantity at least sufiicient to form minimum-boiling azeotropic mixtures with the mono-olefins and the dioleflns, subjecting the mixture to fractional distillation to obtain as an overhead traction a minimum-boiling azeotropic mixturecontaining most of the mono-uleflns present in the fraction treated, and continuing the distillation to obtain a second overhead fraction predominating in a minimum-boiling azeotropie mixture of the added methyl formate and the diolefins.

8. A. process of concentrating diolefins in a mixture consisting of five-carbon atom hydrocarbons'and containing dioleflns and mono-olefins which comprises adding methyl formate to said mixture, said compound being added in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with the *mono-oleflns and the dioleflns in the mixture, and subjecting the mixture thus formed to fractional distillation to obtain at least two overhead fractions the first of which predominates in a. minimum-boiling azeotropic mixture of the added methyl formate and the mono-oleflns, while the second of said fractions predominates in a minimum-boiling azeotropic mixture of the added methyl formate and the diolefins.

9. A process of concentrating dioleflns in a hydrocarbon mixture predominating in five-carbon atom dioleflns and mono-oleflns which comprises adding methyl formate to said mixture, saidcom- P und being added in a quantity at least suflicient to form minimum-boiling azeotropic mixtures with all of the mono-olefinic hydrocarbons in the mixture, and subjecting the mixture thus formed to fractional distillation to distill of! as an overhead fraction a minimum-boiling azeotropic mixture the hydrocarbon content of which predominates in mono-oleflns.

THEODORE W. EVANS. RUPERT C. MORRIS. EDWARD C. SHOKAL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

